(1) Field of the Invention
The present invention relates to an image processing technique of detecting a moving object by segmenting the moving object in video, and in particular, relates to an apparatus that detects a moving object based on motion information in video even when the moving object such as a person moves changing its shape.
(2) Description of the Related Art
Research and development on a region extraction technique with which a moving object is detected by extracting a region of the moving object in an image from the image including an image of the moving object (hereinafter referred simply to as “moving object”) has been widely carried out. The technique of extracting a region of a moving object is, particularly when the moving object is a person, a fundamental technique used in common far: focus control and image quality improvement processing for digital video cameras or digital still cameras; safe driving support system for vehicles; or a collision-avoidance control or a collision-avoidance alarm for a robot with people.
The technique of extracting a region of a moving object in an image includes the following two general methods: (1) a method of identifying a region of the moving object by evaluating the similarity between a moving object model prepared in advance and a candidate region in the image; and (2) a method of identifying a region of the moving object by segmenting the image into plural sub regions to calculate an amount of characteristics and integrating similar regions on the basis of the amount of characteristics.
The former method includes, as a representative method, a method of extracting a candidate of a moving object region from an image, and then evaluates the similarity between the candidate of the moving object region that has been extracted and a moving object model prepared in advance to extract a region with high similarity as the moving object region. There further is a method that utilizes a moving object model in consideration of change in shape when extracting a region of a moving object that moves changing its shape such as a walking person.
According to a method described in Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 8-214289), for example, a silhouette image of a moving object is extracted from plural images, as a candidate of a moving object region. Then, the similarity between a model related to a change in shape of the moving object that has been parameterized in advance and the silhouette image that has been extracted is evaluated, so that a parameter of a region with high similarity and the model are estimated. This makes it possible to apply the parameterized model also to the person that moves periodically changing shape, thereby allowing extraction of a region of the moving object.
The latter method includes, as a representative method, a method, of segmenting once an image into plural sub regions, extracting an amount of characteristics based on a brightness value of an pixel in each of the sub regions, and then evaluating the similarity of the amounts of characteristics between the plural sub regions, to integrate the regions with high similarities as the same moving object region.
According to the method described in Patent Literature 2 (Japanese Unexamined Patent Application Publication No. 2006-031114), for example, an image is segmented once into rectangular sub regions, the amount of characteristics is calculated based on the brightness or motion of the sub regions, and the sub regions are integrated based on an order of the similarities of the amount of characteristics, thereby allowing extraction of the region of a moving object.
In the conventional techniques of region extraction described above, however, there is a problem that a moving object cannot be accurately extracted in the case where the moving object is partially occluded by another moving object in such a scene in which plural moving objects such as people come and go in the street, for example, or in the case where a moving object that is significantly different in size is included.
With the conventional method of extracting a region using a model prepared in advance as represented by the method described in Patent Literature 1, it is necessary to extract a candidate of a moving object region from an image. In this case, the model obtained by parameterizing the moving object cannot be accurately applied to the candidate of the moving object region unless the candidate of the moving object region is appropriately extracted. In the scene described above, in particular, since the shape or size of the moving object changes significantly, it is difficult to appropriately extract the candidate of a moving object region.
Furthermore, even when the candidate of the moving object region is appropriately extracted, there still remains a problem as described below. Particularly in the case where the moving object is an articulated object such as a person, since the range of changes in an image due to a variety of postures or a size of the moving object is significantly large, a huge number of parameters are required when performing parameterization on the moving object model. This induces an error in applying a model. Thus, there is a problem that the moving object cannot be detected accurately because, for example, plural moving objects are erroneously regarded as one moving object in extracting a region, or a region in which a moving object to be extracted does not exist is erroneously extracted as a moving object.
With the conventional method of extracting a region using the amount of characteristics between sub regions as represented by the method described in Patent Literature 2, two sub regions having the brightness values or motions which are used as the amount of characteristics and which are not similar to each other are separated as different regions. Thus, there is a problem that the moving object cannot be detected accurately because, when the brightness value of a moving object differs depending on a position and the motion differs depending on the position, as in a person, two sub regions in the same moving object are extracted as regions belonging to moving objects different from each other.
Therefore, the present invention has been conceived in view of solving the aforementioned conventional problems, and it is an object thereof to provide a moving object detection method and so on capable of extracting a region of a moving object with a high accuracy without being affected by a change in shape or size or occlusion of the moving object in video.